Fossils-extinction and survival-a visual guide

From MediaWiki
Jump to navigation Jump to search

Throughout geological time different organisms have come into existence, lived for a span of time and become extinct. During some periods of time, evolutionary radiations were rapid with numerous new species, genera, families etc. appearing in a geologically short period of time. But at other times there were events that resulted in mass extinction, when a high percentage of organisms suddenly disappeared. There have been five major and eight lesser mass extinction events, which seem to take place every 26 million years. However, they are not usually a single event, but stepwise extinctions over a geologically short period of time, perhaps a few million years. It has been argued that the events are driven by extra-terrestrial activity such as periodic impacts of large asteroids, changes in climate, glacio-eustatic changes in sea level or deep-earth activity such as major volcanic episodes. However, the triggering mechanisms behind these periodic mass extinction events remain conjectural.

The Ediacaran faunas, which included Charnia masoni, went into decline at the end of the Precambrian (P550159)

Perhaps the first extinction event was in the latest Precambrian. This was the time when the early multicelled animals of the the Ediacaran faunas, including jellyfish-like organisms (Ediacaria and Eoporpita), ‘sea pens’ (Charnia and Charniodicus) and worm-like animals (Spriggina, Dicksonia and Kimberella) went into a terminal decline.

Brachiopods such as Lingulella became common in early Cambrian times (P549568)

A huge number of new organisms evolved during the “Cambrian Explosion”. At the beginning of the Cambrian, many organisms began to lay down shells and skeletons. As it is the organism with hard parts that stand the best chance of becoming a fossil, only in Cambrian and younger rocks fossils become common. It is likely that this fact skews the fossil record and the “Cambrian Explosion” probably began in the late Precambrian. However, early in the Cambrian, organisms such as bivalves, corals, brachiopods and trilobites began to evolve and diversify.

Ordovician trilobites like Neseuretus were badly effected by the Ordovician Mass Extinction Event (P549553)

The next mass extinction event, the first of the Phanerozoic, struck in two pulses. It took place towards the end of the latest Ordovician (about 445 million years ago), when about 22% of all families became extinct. About 65% of all the Ordovician trilobites families and 30% of brachiopod families (about 65% of genera) disappeared by the end of this event and echinoderms (echinoids and crinoids) and corals also suffered.

Monograptus evolved in the early Silurian (P549555)

Nearly all the graptolites were wiped out in the Ordovician Mass Extinction Event, probably as a result of the cooling climate and a time of glaciation. However, a few species survived the adverse conditions and during the early Silurian monograptids evolved and diversified.

Devonian fish, particularly the acanthodians and placoderms, suffered during the Devonian Mass Extinction Event (P549493)

During the Devonian Mass Extinction Event (about 355 million years ago) stromatolitic and tabulate coral reefs were badly hit. In low latitudes, 96% of shallow water and 60% of deeper water rugose corals species became extinct. At the base of the food chain, approximately 90% of phytoplankton genera disappeared and this had repercussion higher in the chain. Large numbers of marine fish went into extinction at this time including 65% of placoderms and nearly 90% of acanthodians, although “only” 25-30% of freshwater fish became extinct. Terrestrial plant life appears to have suffered very little.

Spirifer and other types of brachiopods began to evolve in the aftermath of the Devonian Extinction Event (P549532)

Brachiopods had reached their maximum diversity during the Devonian period, but during the Devonian Mass Extinction Event, about 86% of the brachiopod genera went into extinction, noteably in high latitudes. However, after the extinction event, they began to diversify again during early Carboniferous times and a large number of new species began to appear.

Rugose corals, which had been successful during Palaeozoic times, became extinct with the Permian Mass Extinction Event (P549523)

The most severe mass extinction event took place in four pulses at the end of the Permian and beginning of the Triassic, about 248 million years ago. This caused the extinction of 54% of all marine families, representing 83% of all genera and about 96% of all invertebrate species. Tabulate and rugose corals, eurypterids, goniatites, orthid and productid brachiopods, blastoids and trilobites all disappeared, together with several groups of gastropod, crinoid, bryozoa and fish. Shallow marine niches were particularly badly hit.

When the Jurassic seas covered Britain, many new organisms, including crinoids, filled the niches emptied by the Permian Mass Extinction Event (P549504)

Towards the close of the Triassic (around 210 million years ago) about 20% of the families of marine animals disappeared, comprising about 35% of all genera. The ammonoids were almost wiped out. Although only about 8% of bivalve families disappeared, they represented over 40% of all bivalve genera and about 90% of the species. There were other casualties: gastropods lost 13 families, brachiopods 12 families, and sponges 8 families. On land, early plants were replaced by coniferous and bennettitalean species and 13 families of reptiles went into extinction. Britain was landlocked within an arid continent throughout most of the Permian and Triassic times, so that there are few fossils from this time, but on return of the sea at about the Triassic/Jurassic boundary, many new taxa colonised the new and empty ecological niches, including early Jurassic crinoids.

Ammonites, which had been successful throughout the Mesozoic, disappeared during the Cretaceous Mass Extinction Event (P549559)

The most famous mass extinction event took place at the end of the Cretaceous, about 65 million years ago, when the dinosaurs, ammonites and belemnites went into extinction. Several causes that have been postulated, including a period of intense volcanic activity, such as that which extruded the Deccan Traps of India, and an asteroid impact, possibly in central America, that left its signature in a layer of iridium-rich clay and shocked quartz. About 35% of marine genera disappeared at this time, although extinctions actually began earlier in the Maastrichtian stage and continued through to the earliest Palaeogene. This represents several million years and not a single catastrophic event. Dinosaurs, ammonites and belemnites were in rapid decline during the latest Cretaceous, and may have gone into extinction anyway.

Ammonites, Gastropods survived the Cretaceous Mass Extinction Event and were very successful in colonising the oceans and the land (P550292)

It is surprising that some organisms became extinct, yet others were hardly affected. Although the dinosaurs disappeared at the end of the Cretaceous, other reptiles such as turtles, crocodiles and snakes did not. Neither did the birds, which evolved from the dinosaurs. Other organisms took advantage of the extinction event and mammals were one of these; they did not become diverse until after the dinosaurs had disappeared, despite the fact that they had evolved at about the same time. Molluscs such as ammonites and belemnites went into extinction, whereas bivalves and gastropods (also molluscs) survived and proliferated. Gastropods, for example, are now abundant in both aquatic and terrestrial environments and are found from 5 km. below sea level to 6 km. above sea level. But perhaps the most profound mass extinction event is with us today. Human activities have resulted in the loss of environmental niches; modern agricultural techniques have sterilized huge areas for wildlife; industries, cars and aeroplanes generate greenhouse gasses causing global warming and rising sea levels. Human activity has thus changed the environment and caused extinction. Animals, plants and other organisms are going into extinction at an alarming rate. Although it is too late for the extinct organisms, can the organisms currently in decline and in grave danger be saved? There are some difficult economic and environmental questions to be addressed.

Retrieved from "http://earthwise.bgs.ac.uk/index.php?title=Fossils-extinction_and_survival-a_visual_guide&oldid=52453"